One conventional technique for sharing data between electronic devices is the use of a light emitting diode (LED) and a photodetector. For example, devices such as personal digital assistants, mobile telephones, and laptop computers may exchange information if each has a transceiver with an LED and a photodetector.
Internally, conventional transceivers have a receiver circuit, a transmitter circuit, and a signal processor circuit. The transceiver has, on one of its external faces, a photodetector for reception and an LED under a lens for transmission. Prior art FIG. 1 shows a graph 100 of exemplary simulated light intensity profiles for an LED under a spherical lens. The graph 100 shows a horizontal light intensity (HA) curve 102 for light measured along an axis horizontal with respect to the lens and a vertical light intensity (VA) curve 104 for light measured along an axis vertical with respect to the lens. The HA curve 102 and VA curve 104 illustrate the light intensity at an arbitrary distance from the lens over angles between −90 to 90 degrees. The zero degree angle refers to a line that is straight out from the lens and through its central axis. FIG. 1 shows that both the HA curve 102 and the VA curve 104 peak at the central axis (0 degrees). Moreover, the half intensity point of each curve is more than 15 degrees from the central axis. Thus, the device provides good intensity over a cone whose sides extend at an angle of 15 degrees from the central axis.
However, there exists a need to transmit more information than a single LED can convey. In prior art approaches, two different LEDs are placed on the same external face of the transceiver. One such prior art approach is to place an additional lens on the transceiver to accommodate the additional LED. This solution provides a good light intensity profile, such as the one depicted in prior art FIG. 1. However, space is very limited on the transceiver and the solution of adding another lens on the external face for the second LED is undesirable because the devices that use the transceiver are often extremely small. A second prior art approach is to mount two LEDs under a single spherical lens. However, this results in an undesirable light intensity profile. Prior art FIG. 2A shows two LEDs 210a and 210b that are mounted along a vertical axis (axis not shown) under a single spherical lens 215 and the resulting light intensity profiles 220. The light from each of the LEDs 210a, 210b is asymmetrical with respect to the central axis and peaks well off the central axis 240. For example, for LED 210a the light intensity peaks at about +15 degrees from the central axis 240, and for LED 210b the light intensity peaks at about −15 degrees from the central axis 240.
Prior art FIG. 2B illustrates a graph 250 of light intensity for LED 210a of prior art FIG. 2A. The light intensity of the VA curve 252 peaks at more than 15 degrees from the central axis and is relatively low at the central axis (zero degrees). That is, the intensity is less the one half the peak intensity. Moreover, the horizontal curve 254 shows a light intensity that is relatively weak at the central axis, due the fact that the LED is not centered on the central axis. Further, the cone around the central axis is undesirable because of the very low intensity of the VA curve 252 more than −5 degrees from the central axis and the very low intensity of the HA curve 254 more than 10 degrees either side of the central axis.
To comply with standards and to insure successful data transfer, the light intensity must be at a specified level in a cone around the central axis. The required light intensity is technology dependent. It is possible to increase the current supplied to the LED(s) to increase the light intensity everywhere. However, this consumes considerable extra power. As many of the devices for which the transceiver is targeted are battery operated, power consumption is a major issue.
Thus, one problem with conventional LED communication devices having at least two LEDs is that too much space is consumed on a face of the device. Another problem with such devices is that the light intensity profile is too weak near the central axis of the lens and hence data transmission suffers. A still further problem with such devices is that they consume considerable power in overcoming light intensity deficiency problems.